This invention relates to the production of strontium compounds from brine that contains dissolved strontium salts and especially to the recovery of strontium as a sulfate from aqueous brine solutions which also contain a very high proportion of dissolved calcium salts. Strontium sulfate is a valuable product that is useful in the production of, among other things, pyrotechnics, ceramics, glass, and paper.
In the past it has been difficult to extract strontium values from brine that also contains certain alkaline earth salts, particularly calcium. Past methods that have attempted to solve this problem have been, to a great extent, unsatisfactory. For example, U.S. Pat. No. 1,831,251 teaches a method whereby strontium chloride is separated from brines that contain calcium chloride and magnesium chloride by cooling the brine to a temperature below 31.degree. C., or just short of the saturation point of calcium chloride, while agitating. However, this method is disadvantageous in that a large amount of CaCl.sub.2 precipitates along with SrCl.sub.2, so that in the precipitate there is a 20 to 1 ratio of CaCl.sub.2 to SrCl.sub.2. Therefore, in order to separate the CaCl.sub.2 from the SrCl.sub.2 this method would require extensive purification of the precipitate and a high rate of calcium chloride recycle.
U.S. Pat. No. 3,029,133 teaches a method of obtaining SrSO.sub.4 from an inland brine that contains strontium and calcium salts. In the process, the inland brine is evaporated until most of the NaCl in the brine crystallizes out of solution; the brine is then cooled to 70.degree. C. whereupon some tachydrite (2 MgCl.sub.2.CaCl.sub.2.12 H.sub.2 O) crystallizes out. The brine is then further cooled to 28.5.degree. C., at which temperature carnallite (KCl.MgCl.sub.2.6H.sub.2 O) crystallizes out with a substantial portion of the SrCl.sub.2. The carnallite crystals containing the SrCl.sub.2 are then washed with water to produce a brine containing calcium and strontium values with a Ca.sup.++ /Sr.sup.++ molar ratio of approximately 2.7/1. A soluble sulfate is then admixed with this brine and the solution is heated. The reaction mixture of brine and soluble sulfate is then filtered to separate the impure SrSO.sub.4 thus formed which is then leached with a mineral acid. Thus, the method of U.S. Pat. No. 3,029,133 requires several treatments of the brine in order to reduce the molar ratio of Ca.sup.++ /Sr.sup.++ to below 20/1 and preferably below 7/1.
The present invention can be employed to extract strontium from brine that contains a Ca.sup.++ /Sr.sup.++ molar ratio greater than 20/1 and therefore the initial steps of the process of U.S. Pat. No. 3,029,133 can be eliminated. Thus, brines containing high ratios of calcium to strontium values can be directly treated to recover strontium compounds.